The present invention relates to a point-to-multipoint connection system used in a switching system of a broadband ISDN (Integrated Services Digital Network) in the ATM (Asynchronous Transfer Mode).
With the increasing world-wide use of data communication, other important data as well as voice data are being transmitted through public lines. Therefore, high quality data transmission and switching methods are required in future communication networks.
The broadband ISDN (B-ISDN) is a practical step in realising a communication service network for accommodating high-bit-rate data of animation images, etc. as well as voice and lower-bit-rate data, and all the interfaces in the B-ISDN are being standardized. In the B-ISDN, unlike in conventional switching systems, the ATM is used to collectively process data having different bit rates, such as voice data, low-bit-rate data animation images, etc. Besides, the B-ISDN can collectively process serial information and burst information.
In the ATM network, pieces of information in different bands are formed in fixed length data units called cells, not in conventional variable length data units called packets, and then they are transmitted and switched. Thus, in the ATM network, line data and packet data are not separated but can be collectively multiplexed in one-cell units. Therefore, data can be transmitted flexibly at a different transmission speed, and the transmission line can thus be used efficiently.
In ATM communication, subscriber data are divided into a number of parts depending on their length, provided with a header, and put into a cell. The header part of a cell comprises several bytes and its data part comprises several tens of bytes. A header is provided with a channel identifier indicating a subscriber of data contained in the cell. After subscriber information is put into a cell, the cell is multiplexed in an ATM highway, transmitted, and switched. In a switching unit, a cell is inputted from each input highway to a switch matrix, and outputted to a target output highway by being autonomously switched by each switch in the switch matrix according to the channel identifier added to the cell. (Actually, information called a tag is used).
Accordingly, a cell inputted from an input highway is outputted to one of a plurality of output highways through a switch matrix of an ATM switch. Thus, in the ATM switching method, point-to-point communication is established as a basic communication method.
However, considering that a B-ISDN using the ATM must be a switching system for efficiently processing animation data, etc. as well as voice data, point-to-multipoint communication services are required, for example, for television conferences, cable television services (CATV), etc.
The first system in the prior art technology for realizing point-to-multipoint connection is capable of establishing a point-to-multipoint connection using a switch matrix of an ATM switch.
However, in the first system, determination must be comprehensively made as to where in a switching unit a cell is copied for a point-to-multipoint connection, and the idle/busy state of each channel (time slot) used in a switching unit must be confirmed. As a result, the switching process necessarily becomes complex, thereby losing the B-ISDN's merit that collective communication can be performed at a high bit rate using a simple protocol.
In the first system, the number of channels (time slots) increases with the number of copies (distributions). Therefore, an ATM switch itself must be capable of processing increased channels. However, since a switch matrix of the ATM switch requires a hardware function for autonomously switching a cell, an additional point-to-multipoint function requires a complex hardware mechanism, thereby reducing both maintenance and operation efficiency.
The second system of the prior art technology realizes a point-to-multipoint communication service by copying an inputted cell and inputting a plurality of resultant cells to the ATM switch using a trunk provided externally for an ATM switch and exclusively used for a point-to-multipoint connection.
The second system is effective in services such as television conferences, in which information is distributed to only a limited number of subscribers. However, in such services as CATVs, where information is distributed to a number of general subscribers, it causes a problem in that the above described trunk is required for each subscriber, a large-scale switching unit must be provided, the necessary process becomes complex, and the cost greatly increases.
Besides, in a common switching network in which a central office and remote stations are connected and subscriber lines are concentrated at each remote station, the above described first and second systems of the prior art technology transmit copies of information indicating the number of subscribers between a central office and remote stations, thus resulting in wasteful transmission.